1. Field of the Invention
The present invention relates to a magnetic-resonance imaging apparatus.
2. Summary of the Prior Art
In a magnetic-resonance imaging apparatus, a strong uniform magnetic field is generated in a predetermined volume, and then a magnetic field gradient is generated within that volume to induce magnetic resonance in an object to be investigated. The resonance is then analyzed to determine the properties of the object. Such a magnetic-resonance imaging apparatus finds particular applicability for medical purposes as it represents a non-invasive method for investigating the interior of a human body.
In order to generate the magnetic field gradients, appropriate coils are supported within the volume in which the strong uniform magnetic field is created, and it is normal to support those coils by a support known as a bobbin, which is in the form of a hollow cylinder, so that the coils may be secured to the inner or outer cylindrical surface thereof. The bobbin is usually made of epoxy resin having a very high elastic modulus. Examples of such magnetic-resonance imaging apparatuses are disclosed in, for example, JP-A-62-261105, JP-A-62-229906, JP-A-62-239503 and JP-A-63-158047.
In order to generate suitable magnetic gradients for magnetic resonance imaging, current pulses are applied to the coils supported on the bobbin. Hence those coils are current carriers in the uniform magnetic field, and forces are generated on the coils, the directions of which forces are determined by Fleming's left hand rule. Thus, one part of each of the coils will experience a force in one direction and another part will experience force in the opposite direction. Therefore, a pulsating load is applied to the bobbin. When the uniform magnetic field is large, the forces can set up oscillations of the bobbin and so generate noise. For medical magnetic resonance imaging, part of the human body to be investigated must be located within the bobbin and, the noise can produce discomfort.
Therefore, JP-A-61-279238 proposed that a rubber pad be interposed between the bobbin and each coil which generated the magnetic field gradient. When the current in a coil generated forces, the coil could move relative to the bobbin, due to deformation of the pad and the vibrations thus generated were damped by the rubber pad.